When does a MME trigger UE context modification procedure towards an eNodeB for an UE ?

Question

From MME to eNodeB, UE context modification procedure is triggered I know but what makes trigger of UE Context modification procedure from eNodeB to MME ?

Answer 1

As part of CS Fallback, MME sends a UE Context Modification Request message to eNB indicating that the UE should be moved to UTRAN/GERAN. The eNB may optionally solicit a measurement report from the UE to determine the target GERAN/UTRAN cell to which the redirection procedure will be performed.

You can find a detailed explanation in the following link -
http://www.lteandbeyond.com/2013/09/CSFB-Mobile-Terminating-Call-in-Active-mode.html

Hope this helps !

Answer 2

Mobile terminating cs fall back procedure at that time MSC txts using SGs interface.MME fowards ue context modification request msg that includes the cs fallback indicatorit tells mobile move to 3g cell or 2g cell